[PATCH] drm/bridge: anx7625: Fix edid_read break case in sp_tx_edid_read()

[PATCH] drm/bridge: anx7625: Fix edid_read break case in sp_tx_edid_read()

[Hsin-Yi Wang]

edid_read() was assumed to return 0 on success. After
7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()"),
the function can return >= 0 for successful case. Fix the g_edid_break
condition in sp_tx_edid_read().

Fixes: 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
---
 drivers/gpu/drm/bridge/analogix/anx7625.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
index 1a871f6b6822ee..392203b576042b 100644
--- a/drivers/gpu/drm/bridge/analogix/anx7625.c
+++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
@@ -826,7 +826,7 @@ static int sp_tx_edid_read(struct anx7625_data *ctx,
 				g_edid_break = edid_read(ctx, offset,
 							 pblock_buf);
 
-				if (g_edid_break)
+				if (g_edid_break < 0)
 					break;
 
 				memcpy(&pedid_blocks_buf[offset],
-- 
2.34.0.rc1.387.gb447b232ab-goog

Re: [PATCH] drm/bridge: anx7625: Fix edid_read break case in sp_tx_edid_read()

[Robert Foss]

Hey Hsin-Yi,

Thanks for catching this.

checkpatch --strict is throwing an error for this patch. With that,
feel free to add my r-b.

Reviewed-by: Robert Foss <robert.foss@linaro.org>

On Fri, 12 Nov 2021 at 12:24, Hsin-Yi Wang <hsinyi@chromium.org> wrote:
>
> edid_read() was assumed to return 0 on success. After
> 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()"),
> the function can return >= 0 for successful case. Fix the g_edid_break
> condition in sp_tx_edid_read().
>
> Fixes: 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()")
> Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
> ---
>  drivers/gpu/drm/bridge/analogix/anx7625.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> index 1a871f6b6822ee..392203b576042b 100644
> --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> @@ -826,7 +826,7 @@ static int sp_tx_edid_read(struct anx7625_data *ctx,
>                                 g_edid_break = edid_read(ctx, offset,
>                                                          pblock_buf);
>
> -                               if (g_edid_break)
> +                               if (g_edid_break < 0)
>                                         break;
>
>                                 memcpy(&pedid_blocks_buf[offset],
> --
> 2.34.0.rc1.387.gb447b232ab-goog
>

Re: [PATCH] drm/bridge: anx7625: Fix edid_read break case in sp_tx_edid_read()

[Guenter Roeck]

On Fri, Nov 12, 2021 at 07:24:33PM +0800, Hsin-Yi Wang wrote:
> edid_read() was assumed to return 0 on success. After
> 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()"),
> the function can return >= 0 for successful case. Fix the g_edid_break
> condition in sp_tx_edid_read().
> 
> Fixes: 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()")
> Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
> Reviewed-by: Robert Foss <robert.foss@linaro.org>
> ---
>  drivers/gpu/drm/bridge/analogix/anx7625.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> index 1a871f6b6822ee..392203b576042b 100644
> --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> @@ -826,7 +826,7 @@ static int sp_tx_edid_read(struct anx7625_data *ctx,
>  				g_edid_break = edid_read(ctx, offset,
>  							 pblock_buf);
>  
> -				if (g_edid_break)
> +				if (g_edid_break < 0)
>  					break;

g_edid_break is declared u8 and thus never < 0. The break; statement
doesn't indicate an error but that a break was encountered, and the
value of g_edid_break == 1 is used elsewhere in the function to indicate
that condition.  It also doesn't break out of the outer loop, but only
out of the switch statement.

With this patch in place, g_edid_break will have a more or less random
value after an error, and the behavior of this function will be undefined.

Guenter

Re: [PATCH] drm/bridge: anx7625: Fix edid_read break case in sp_tx_edid_read()

[Hsin-Yi Wang]

On Fri, Nov 19, 2021 at 12:52 AM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On Fri, Nov 12, 2021 at 07:24:33PM +0800, Hsin-Yi Wang wrote:
> > edid_read() was assumed to return 0 on success. After
> > 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()"),
> > the function can return >= 0 for successful case. Fix the g_edid_break
> > condition in sp_tx_edid_read().
> >
> > Fixes: 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()")
> > Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
> > Reviewed-by: Robert Foss <robert.foss@linaro.org>
> > ---
> >  drivers/gpu/drm/bridge/analogix/anx7625.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> > index 1a871f6b6822ee..392203b576042b 100644
> > --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> > +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> > @@ -826,7 +826,7 @@ static int sp_tx_edid_read(struct anx7625_data *ctx,
> >                               g_edid_break = edid_read(ctx, offset,
> >                                                        pblock_buf);
> >
> > -                             if (g_edid_break)
> > +                             if (g_edid_break < 0)
> >                                       break;
>
> g_edid_break is declared u8 and thus never < 0. The break; statement
> doesn't indicate an error but that a break was encountered, and the
> value of g_edid_break == 1 is used elsewhere in the function to indicate
> that condition.  It also doesn't break out of the outer loop, but only
> out of the switch statement.
>
> With this patch in place, g_edid_break will have a more or less random
> value after an error, and the behavior of this function will be undefined.
>

Before 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from
sp_tx_rst_aux()"), edid_read() only returns 0 (i2c read bytes > 0) or
-EIO, so g_edid_break also couldn't be 1. After checking the code
more, I guess what it wants to do is:

- For the 1st and 2nd block (case 0 and 1) in sp_tx_edid_read(), copy
the buffer on success.
- For other blocks (case 2 and 3), process the buffer when the last
g_edid_break updated in previous blocks is not 1. These blocks
probably don't need to be updated if previous blocks have failed. The
whole edid will be checked if valid.

I will update the patch.

> Guenter